LAF：一部分A320特有的“巡航减震系统”
提问
对于A320机型，减载功能和载荷保护是不一样的，对吗？它们的区别是否在于减载是感受到外力颠簸之后副翼或者扰流板向上偏转来减小受力，而载荷保护是感受到内力侧杆？
如果我们要对比两个概念，那么第一步是把概念精确表达出来。所以我想首先梳理一下，提问里的“减载功能”指的是A320 FCOM DSC-27-20-10-40 Load Alleviation Function（减载功能），而“载荷保护”指的是DSC-27-20-10-20-A-00001073.0001001 LOAD FACTOR PROTECTION（载荷系数保护）。这样的话，我们通过对比FCOM的原文就可以知道了：这两者确实是不一样的。
顺带说一点儿“翻译”的问题。Load Alleviation Function，缩写LAF，中文版通常翻译为“减载功能”，但我建议把 alleviation 的意思翻译出来，至少是理解到位了。中文里的“镇痛、纾困”也用这个词，它实际上表示缓解。所以是否考虑“载荷减缓功能”？翻译很麻烦，但更重要的是理解到位，因此建议把中英文都掌握了。LOAD FACTOR PROTECTION，建议翻译成“载荷系数保护”，factor在CCAR规章里翻译成系数，很多FCOM中文版写成了“因数”或“因子”，也不妨碍理解，但是为了掌握好概念，不要把factor整个抹掉了。
回到正题。LAF是一部分A320特有的功能（在FCOM章节里可以通过MSN号或者注册号来确定；很多人可能都不知道它），它是为了保护大翼结构而设计，LAF指令只在飞机光洁构型且处于正常法则时会有，但它不是正常法则这个概念本身所指的5种保护之一。它的起作用，与阵风算法、垂直加速度以及侧杆的俯仰指令有关系。形象理解的话，它应该是主要用于巡航阶段的一种减弱飞机颠簸程度的“减震系统”。
而载荷系数保护是所有飞机都有的，是为了保护整个机身的结构受力。它是正常法则这个概念的5种保护之一。无论是否在光洁构型，它都可能起作用，例如在起降阶段。它和侧杆输入当然也是有关系的，但不是直接关系；有直接关系的是由侧杆操作导致的垂直加速度。这个参数不是相对于地面的垂直，而是指机身垂直轴上的加速度，一些资料里写成“Nz指令”。
载荷系数保护通常不是单独起作用，而是和正常法则的其他几种保护同时起作用，一组计算机同时实现这些不同的保护功能；单单就载荷系数保护来说，计算机的目标是确保飞机的垂直加速度在 2.5 g到-1 g之间（光洁形态）或 2 g到 0之间（襟翼放出时）。形象理解的话，它应该是全程起作用的“加速度限制系统”。
关于这两种功能，FCOM有详细的说明。非常严肃地建议：除了中文版，也一定看看英文版，有助于准确掌握概念。同时也推荐下面这篇《带G上天》，它是对“载荷系数保护”功能的一个解读。
往期推荐
附上来自AMM里关于LAF的干货。内容和FCOM一致，只是更详细。没有困难词汇，不翻译啦。
Load Alleviation Function (LAF)
(1) General
The load alleviation function is used to alleviate the loads on the wings.
This is done through the upward deflection of the control surfaces:
· The two ailerons only, or
· The two ailerons associated with spoilers 4 and 5.
The LAF has two sub-functions:
· One that is activated during a stable maneuver (aileron deflections only)
· One that is activated during discrete gust detection (aileron and spoiler deflections).
The LAF is available in clean configuration and in normal law only, with the autopilot engaged or not.
The LAF orders are added to those generated by the normal law.
The LAF generates surface deflection orders in relation to load factors from:
· Gust,
· Acceleration because of a bank,
· Acceleration because of a side stick order in pitch.
(2) LAF (maneuver function) (deflection of the ailerons only)
(a) Control surfaces
The LAF (maneuver function) uses the two ailerons only with an upward deflection of 15° maximum (phased out with the speedbrake deflection).
(b) Inhibition conditions
The LAF is not available when:
· The flight control law is not the 'normal law'
· The slat/flap is not in clean configuration
· The wing tip brake is ON
· There is one-elevator configuration if there is a double hydraulic failure (Blue and Yellow)
· There is a failure of one aileron
· There is a double accelerometer failure in ELAC 1.
(c) Deflection logic
The ailerons are deflected in relation to the Nz measured and thresholds. (note: Nz stand for VERTICAL ACCELERATION)
Then, when the ailerons are deflected, they stay deflected for a minimum of 2.5 seconds (hold time).
(3) LAF (discrete gust function) (deflection of the ailerons and spoilers)
(a) Control surfaces
The LAF (discrete gust function) uses:
· The two ailerons with an upward deflection of 10° maximum, and
· Spoilers 4 and 5 with a deflection of 10° maximum
(b) Inhibition conditions
The LAF is not available when:
· The flight control law is not the 'normal law'
· The slat/flap is not in clean configuration
· The wing tip brake is ON
· There is a one-elevator configuration if there is double hydraulic failure (Blue and Yellow)
· There is a failure of one aileron
· There is a double accelerometer failure in ELAC 1.
(c) Deflection logic
The two ailerons and spoilers 4/5 are deflected up when the vertical acceleration is more than 1.3g and the gust detector algorithm is active.
When activated, ailerons and spoilers stay deflected for a minimum of 2.5 seconds (hold time).
参考资料
A320 FCOM AMM